Device for determining a characteristic point in the cardiac cycle

ABSTRACT

The present invention relates to a device for determining a characteristic point in the cardiac cycle that comprises means for calculating the curve of the blood flow rate D(t) in the aorta from the curve of the arterial blood pressure signal P(t) and determining from the curve of the blood flow rate D(t) in each cardiac cycle the time at which an [the] incisura point lies. The device can be used for activating an intra-aortal balloon pump (IABP).

[0001] The invention relates to a device for determining acharacteristic point in the cardiac cycle. Such a device can be usedfor, for example, activating an intra-aortal balloon pump (IABP).

[0002] An IABP contains, inter alia, an intra-aortal balloon (IAB),which can be inserted, for example, into the aorta of a patient with apoorly functioning heart, and a pumping device.

[0003] In each cardiac cycle the IAB is inflated by means of the pumpingdevice after the end of an ejection phase of the left ventricle of theheart, and is deflated again before the commencement of the followingejection phase.

[0004] The pumping action of the heart is improved in this way, andthere is an improvement in the blood supply to the coronary artery.

[0005] For good functioning of the IABP it is of great importance forthe IAB to be inflated and deflated at the correct times in the cardiaccycle. In particular, the correct choice of the time at which the IAB isinflated is of very great importance.

[0006] If the IAB is inflated too soon, the pumping action of the heartis reinforced to a lesser extent, or the pumping action can even beadversely affected, because the prematurely inflated IAB causes a flowresistance in the aorta during the ejection of the left ventricle whichis still occurring at the time.

[0007] If the time selected is too late, the functioning of the IABP isalso less effective. A lover volume of blood is then pumped through theIAB, and the coronary artery and the vascular bed undergo a highperfusion pressure for only a short period of time.

[0008] The times for inflating and deflating the IAB can be set manuallyby an experienced person at fixed times in the cardiac cycle on thebasis of the electrocardiogram (ECG) of the heart. A disadvantage ofthis is that when there is a gradual acceleration or slowing-down of thecardiac cycle the set times deviate increasingly from the desired times,and therefore have to be reset repeatedly. It is also impossible to makeallowances for an irregular cardiac cycle, and in particular the settingof the time at which the IAB is inflated is not performed sufficientlyaccurately.

[0009] The end of the ejection phase and the accompanying closure of theaortic valve are themselves indicated accurately by the occurrence of adip in the arterial blood pressure signal P(t). This dip is also knownas the incisura point.

[0010] U.S. Pat. No. 5,183,051 discloses a device by means of which anattempt is made to determine the incisura point by looking for the dipin the curve of the arterial blood pressure signal P(t) within apreviously defined period of time. However, the period of time may beincorrectly defined and, besides, the device does not work in the caseof a damped blood pressure signal, because in that case the incisurapoint is not accompanied by a clear blood pressure change.

[0011] A further disadvantage of this device is that it is still notpossible to make allowance for an irregular cardiac cycle, whilepatients in whom an IABP is used generally have an irregular cardiaccycle. Moreover, the use of the device for activating an IABP is notmentioned at all in U.S. Pat. No. 5,183,051.

[0012] A device which detects the incisura point in the curve of thearterial blood pressure signal P(t) is proposed in IEEE Transactions onBiomedical Engineering 1990, 37(2), pp. 182-192. However, it is possiblethat this device may interpret irregularities in the curve of thearterial blood pressure signal as the incisura point, which upsets thefunctioning of the IABP.

[0013] U.S. Pat. No. 4,809,681 discloses a device for activating an IABPwhich determines from the ECG the point at which the IAB must bedeflated. However, it is not possible to determine the incisura pointusing the device.

[0014] Sakamoto et al., ASAIO Journal 1995, pp. 79-83, discloses adevice which forecasts the position of the incisura point in a cardiaccycle from the ECG by calculating the length of the ejection phase fromthe period of time of the previous heartbeat. This device is stillinaccurate.

[0015] The object of the invention is to provide a device fordetermining a characteristic point in the cardiac cycle which does nothave the above-mentioned disadvantages.

[0016] Surprisingly, this is achieved by the fact that the deviceaccording to the invention comprises means for calculating the curve ofthe blood flow rate D(t) in the aorta from the curve of the arterialblood pressure signal P(t) and determining the time (ti) at which theincisura point lies from the curve of the blood flow rate D(t) in eachcardiac cycle.

[0017] The time at which the incisura point lies can be determinedinstantaneously from the curve of D(t), so that the moment the incisurapoint is reached in a cardiac cycle the IABP can be activated by thedevice at precisely the correct moment (in real time).

[0018] A further advantage of the device according to the invention isthat only the curve of the arterial blood pressure need be measured,which is a simple procedure.

[0019] The device according to the invention can be used for manypurposes. For instance, the device is suitable for use in activatingheart-function-supporting equipment. The device is preferably used foractivating an IABP. It is also possible to use the device as part of amonitoring system which determines the duration of the ejection phase ofa heartbeat and relates the latter to the total period of the heartbeat.The haemodynamic condition of a patient can be followed using such amonitoring system. This can be carried out starting from the arterialblood pressure signal or from the pulmonic blood pressure signal.

[0020] The device preferably has means for delivering a signal at themoment then the incisura point is reached. By means of said signal, theIABP, for example, is put into operation in order to inflate the IAB.The means which the device according to the invention comprises forcalculating the curve of the blood flow rate D(t) from the curve of thearterial blood pressure signal P(t) can be a calculating device, forexample a computer, a microprocessor or a digital calculating machine.The calculating device in this case is loaded with a calculation programfor calculating the blood flow rate D(t) from the arterial bloodpressure signal P(t).

[0021] The calculation program can be based on one of the models knownto the person skilled in the art for calculating the blood flow rateD(t) from the arterial blood pressure signal P(t).

[0022] Examples of such models are given in IEEE Transactions onBiomedical Engineering 1985, 32(2), pp. 174-176, Am. J. Physiol. 1988,255 (Heart Circ. Physiol.), H742-H753 and in WO 92/12669.

[0023] Very good results are obtained if the calculation program isbased on the Windkessel model, as also described in the abovementionedliterature. In a suitable embodiment the Windkessel model is based onthree elements, namely a characteristic input resistance, Rao, anarterial compliance, Cw, and a peripheral resistance, Rp.

[0024] The characteristic input resistance, Rao, represents the flowresistance experienced by the heart. The arterial compliance, Cw,represents the ability of the aorta and the arteries to store aparticular volume of blood as the result of elastic expansion. Theperipheral resistance, Rp, represents the resistance of the vascularbed.

[0025] The values used for the elements in the Windkessel model areknown from the literature. Suitable values are known from, for example,Am. J. Physiol. 1988, 255 (Heart Circ. Physiol.), H742-H753.

[0026] Very good results are achieved if account is taken of thedependence of the elasticity of the aorta on the current blood pressure,as described in WO 92/12669.

[0027] An advantage of the calculation program based on the Windkesselmodel is that the values for the elements in the model used in thecalculation do influence the absolute value of the calculated blood flowrate, but the position of the incisura point depends only to a verysmall extent on the values used for the elements. It is therefore notnecessary to know the values of the elements very well for a particularpatient in order to obtain good results from the calculation of theincisura point.

[0028] The position of the incisura point can be determined veryaccurately from the curve of the blood flow rate D(t) calculated in thisway, while said point is difficult to determine from the curve of thearterial blood pressure signal P(t).

[0029] It is therefore possible for the first local minimum which occursin the curve of the blood flow rate D(t) after the beginning of theinjection phase of the left ventricle to be determined. This pointrepresents the incisura point.

[0030] The minimum in the blood flow rate D(t) can be determinedaccording to one of the calculation methods known for it. For instance,it is possible in each case to compare three successive values in thecurve of the blood flow rate D(t) with each other. If the conditionD(t−dt)>D(t)<D(T+dt) is met, the minimum is reached at time t. If dt isselected at a sufficiently low level, the minimum can actually bedetected virtually at the moment when it is reached, and at that momenta signal can be generated and supplied to the IABP. It is preferable fordt to be less than 0.02 second, more preferably less than 0.01 second,and still more preferably less than 0.005 second.

[0031] Even more accurate results are obtained if the occurrence of thelocal minimum is also subject to the condition that at the moment whenthe local minimum is reached the blood flow rate is situated below aspecific threshold value Dd. This ensures that reflections in the bloodpressure signal which can occur during the ejection phase of the leftheart ventricle are not detected as the incisura point. The thresholdvalue can be equal to, for example, 10% of the value of the blood flowrate, while the blood flow rate from the left ventricle has reached itsmaximum value. The calculation program can be set up in such a way thatthe threshold value is recalculated after each cardiac cycle. Thethreshold value is preferably selected so that it is equal to zero,Dd=0. This threshold value is reached just before the incisura pointoccurs, so that the chance of a reflection being detected wrongly as anincisura point is very small.

[0032] In order to obtain an accurate calculation of D(t) when thelatter is just above the zero value, it is important to know the correctvalue for Rp. This value can also be calculated from Rao and Cw usingthe Windkessel model, by assuming that the total quantity of blood whichflows into the Windkessel compliance in a heartbeat, or even taken overa number of heartbeats, also flows out of it again. Rao and Cw as suchcan be estimated accurately for a patient if sex and age are known.

[0033] It is also possible for the incisura point to be determined atthe time that D(t)=0, after the beginning of the ejection phase. Thisensures that a signal can already be given to the IABP just before theaorta valves close, and if an inertia occurs during the inflation of theIAB, the IAB can be inflated when the valves are actually closing.

[0034] The beginning of the ejection phase of the left heart ventriclecan be determined from the ECG, from the curve of the arterial bloodpressure signal P(t), or from a combination of the two. The way in whichthis can be carried out is known to the person skilled in the art.

[0035] The time at which the beginning of the ejection phase is reachedcan be transmitted to the device according to the invention, which usesthe time to activate the device according to the invention for thecalculation of the next incisura point. A signal for deflating the IABPcan also be supplied at that time to the IABP.

[0036] A yet further improved device according to the invention isobtained if the device has a filter for filtering high-frequency noiseout of the blood pressure signal.

[0037] This ensures that irregularities in the curve of the bloodpressure signal are filtered out, with the result that the chance of thedevice detecting the incisura point at an incorrect time is reduced evenfurther. This is important in particular if the device has to functionin an environment where its proper functioning can be interfered with bythe frequent occurrence of electromagnetic waves.

[0038] The device according to the invention is preferably connected toa pressure recorder for measuring the arterial blood pressure, whichpressure recorder is attached to the IAB. In this way a blood pressuresignal P(t) is supplied to the device and is measured in the aorta,directly behind the heart. This reduces even further the chance of theblood pressure signal having irregularities which are incorrectlyidentified by the device as an incisura point. Another advantage of thisis that the measured pressure signal is slowed down little, if at all,but properly reflects the current stage of the cardiac cycle.

[0039] The invention is explained in greater detail with reference tothe drawing, without being restricted thereto.

[0040]FIG. 1 gives an example of the curve of a measured arterial bloodpressure P(t). P(t) is plotted in millimeters mercury pressure (mmHg) onthe y-axis, and the time t is plotted in seconds (sec) on the x-axis.

[0041]FIG. 2 shows a diagram of a Windkessel model.

[0042]FIG. 3 gives a diagram of a calculation program for calculatingthe curve of the blood flow rate D(t) from the curve of the arterialblood pressure signal P(t), based on the Windkessel model of FIG. 2.

[0043]FIG. 4 gives the curve of the blood flow rate D(t) calculated fromthe curve of the measured arterial blood pressure signal P(t) accordingto FIG. 1, using the calculation program from FIG. 3. D(t) is plotted inmilliliters per second (ml/sec) on the y-axis, and the time is againplotted on the x-axis.

[0044] In FIG. 1 the time a in the curve of the arterial blood pressuresignal P(t) is the point at which an ejection phase of the leftventricle begins. This is the time at which the IAB must be deflated.Further, at this time a signal can be supplied to the device accordingto the invention, in order to start up a calculation cycle fordetermining the typical point in the cardiac cycle. The time a can bedetermined from the ECG, from the arterial blood pressure signal P(t),or from both.

[0045] The time b is the time at which the incisura point is reached andthe heart valves close. This is the time at which the IAB must beinflated. It can be seen clearly that the incisura point is manifestedonly in the form of an unsharp local minimum in the curve of thearterial blood pressure signal P(t).

[0046] At the time a′ the cardiac cycle ends, and the ejection phase ofthe following cardiac cycle begins. The time interval a-b is also knownas the systolic phase. The time interval b-a′ is also known as thediastolic phase.

[0047]FIG. 2 gives a diagram of a simple Windkessel model containing thefollowing elements; a characteristic input resistance, Rao (1), anarterial compliance, Cw (2), and a peripheral resistance, Rp (3).Further, the aorta valves are modelled by means of an ideal diode (4)which closes after D(t) becomes negative in a cardiac cycle.

[0048]FIG. 3 gives a diagram for a calculation program for calculatingthe incisura point.

[0049] In step 1, indicated by (1) in FIG. 3, the beginning of theejection phase (point a) is detected, for example from the ECG. So longas the beginning of the ejection phase has not yet been detected, D(t)=0is assumed. If the beginning of the ejection phase is detected, oneproceeds to step 2 (2).

[0050] During step 2 the curve of the blood flow rate D(t) is calculatedfrom the measured pressure signal P(t), for example by means of theequation;

(1+Rao/Rp).Iao+Rao.Cw.{dot over (I)}ao=Pao/Rp+Cw.{dot over (P)}ao

[0051] ({dot over (I)}ao and {dot over (P)}ao are the first-orderderivatives according to time of {dot over (I)}ao and Pao)

[0052] As soon as D(t)<0 the program starts to look for the first localminimum in the D(t), for example by in each case comparing a series ofat least three successive points in the curve of D(t).

[0053] When the first local minimum, the incisura point, is reached, asignal is sent to the IABP, and step 1 (1) starts again.

[0054] From the curve of the blood flow rate in FIG. 4, calculated fromthe curve of the arterial blood pressure signal according to FIG. 1 bymeans of the calculation program from FIG. 3, the incisura point can beseen clearly as a sharp local minimum.

[0055] The diode (4) from FIG. 2 makes D(t) equal to zero during thediastolic phase.

1. Device for determining a characteristic point in the cardiac cycle,characterized in that the device comprises means for calculating thecurve of the blood flow rate D(t) in the aorta from the curve of thearterial blood pressure signal P(t) and determining from the curve ofthe blood flow rate D(t) in each cardiac cycle the time at which theincisura point lies.
 2. Device according to claim 1, characterized inthat the means are formed by a calculating device, loaded with acalculation program for calculating the blood flow rate D(t) from thecurve of the arterial blood pressure signal P(t).
 3. Device according toclaim 1 or 2, characterized in that the device has means for deliveringa signal at the moment when the incisura point is reached.
 4. Deviceaccording to one of claims 1-3, characterized in that the incisura pointis determined from the occurrence of the first local minimum in thecurve of the blood flow rate D(t) after the beginning of the ejectionphase.
 5. Device according to claim 4, characterized in that theoccurrence of the first local minimum is also subject to the conditionthat the blood flow rate lies below a threshold value Dd.
 6. Deviceaccording to claim 5, characterized in that Dd is equal to 10% of thevalue which the blood flow rate has while the blood flow rate from theleft ventricle has reached its maximum value.
 7. Device according toclaim 6, characterized in that Dd=0.
 8. Device according to one ofclaims 1-3, characterized in that the incisura point is determined atthe time that D(t)=0 after the beginning of the ejection phase. 9.Device according to one of claims 1-8, characterized in that the devicehas a filter for filtering out the high-frequency noise on the arterialblood pressure signal P(t).